The present invention relates to rail-to-rail input common mode range differential amplifiers, and more particularly to a CMOS input stage with wide common-mode range.
In many areas of the electronics industry, such as the portable electronics industry, designers are turning toward lower operating voltages. This enables electronic circuit designers to design systems with smaller power supplies, which in turn reduces product weight and size, and increases the life expectancy of the system DC power supply.
However, while reduced operating voltages are beneficial in reducing product size and extending useful battery life, the lower voltages typically adversely affect circuit operation. For example, as circuit supply voltages are reduced, the range of circuit signal voltages which are available is also reduced. The reduced range of operating voltages is especially a concern for designers of operational amplifiers, such as differential amplifiers having a differential input stage.
The design and operation of conventional differential amplifiers is well known. The input stage of the differential amplifier typically includes an upper and lower supply rail, where the input stage further comprises one or more differential transistor pairs with associated current source(s). In addition, the input stage has a common-mode input voltage which defines the voltage range within which the input stage of the amplifier operates. The common-mode input voltage may typically be measured between the upper or lower supply rail of the input stage and the gate or base of one of the transistors, and is sometimes called the rail-to-rail voltage.
It is the goal of differential amplifier designers to design an operational amplifier with an input stage capable of operating over the full range of the common mode voltage. As noted above, however, where the circuit supply voltages are reduced, the range of circuit signal voltages over which the input stage of amplifier may operate is also reduced, which deteriorates the overall effectiveness of the amplifier""s operation.
In that regard, designers have attempted to design circuitry such that supply voltages will not be less than the transistor saturation voltage plus the transistor gate to source voltage (e.g., Vsat+Vgs). By manipulating the circuit supply voltage in this manner, the circuit transistors are provided sufficient xe2x80x9cheadroomxe2x80x9d to operate. On the other hand, the effect of reducing the supply voltages is that the gate to source voltage Vgs of the transistor which makes up the input stage of the amplifier will also be reduced.
As used herein, headroom may be defined as the capacity to accommodate an input signal swing without driving an amplifier into saturation or into a non-linear operating region. Where the headroom occupies a larger fraction of the voltage supply range, the available common-mode input voltage range and the operating range of the input stage decreases. That is, where a differential amplifier is forced to extend beyond this narrow range of common-mode voltages, the differential-mode gain may drop off sharply, sometimes to zero. As a result, the differential amplifier may typically operate only over a relatively narrow range of common-mode input voltages.
To extend the common mode operating range, conventional amplifiers are often designed with a rail-to-rail input stage using depletion type transistors. One such rail-to-rail input stage amplifier is currently being produced by ON Semiconductor, Semiconductor Components Industries, LLC as part number MC33502. The MC33502 amplifier uses a single pair of depletion transistors to form a differential input stage. The general operation of the MC33502 part may be understood with reference to FIG. 1, below.
FIG. 1 shows an example of a conventional rail-to-rail input common mode range differential amplifier 10, as described in U.S. Pat. No. 5,808,513 issued Sep. 15, 1998 to Archer (hereinafter xe2x80x9cArcherxe2x80x9d). Archer purports to provide a rail-to-rail common mode range differential amplifier which operates on rail-to-rail voltages down to approximately one volt.
As shown in FIG. 1, amplifier 10 includes a current source 12 that outputs a tail current IT, a differential pair 14 having depletion-type transistors M1 and M2 which output first and second intermediate currents, IM1, and IM2, and an active or passive load that amplifies the difference in magnitude between the first and second intermediate currents IM1, and IM2. In this conventional construction, amplifier 10 may vary the magnitude of the first and second intermediate currents IM1, and IM2 in response to the voltage difference between the differential inputs VIN+ and VINxe2x88x92.
One problem with utilizing depletion-type transistors M1 and M2, however, is that as the supply voltage is reduced, the effective change in the threshold voltage due to the body effect becomes less and less. Thus, for example, where the threshold voltage effectively drops from 0.5 volts to xe2x88x920.5 volts for a rail-to-rail voltage of 3 volts, the effective change for a rail-to-rail voltage of only 1 volt may only be seen as a change of 0.5 volts to 0.1 volts. This, in turn, widens the amplifier headroom and lowers t he common-mode range of the amplifier design.
Further, with respect to the construction of the ON Semiconductor part MC33502 which uses depletion-type transistors, a special production process is required which typically includes supplementing the circuit with additional implants. That is, there is a need to add additional diffuision ion-implanting steps to the process to provide a thin channel-type layer under the gate of the MOS transistor. Consequently, as should be understood, the reduced common-mode range and the added complexity of the special production process make the rail-to-rail differential amplifier constructions of the ON Semiconductor part MC33502 and the Archer patent less desirable to use.
Another well known rail-to-rail input stage differential amplifier construction is described in U.S. Pat. No. 4,555,673 issued Nov. 26, 1985 to Huijsing et al. (hereinafter xe2x80x9cHuijusingxe2x80x9d). Huijusing purports to describe an amplifier that operates between first and second supply voltages where the range for the power supply voltages, VPS, is divided into three sub-ranges. A first end sub-range of the invention is described as extending from the first supply voltage to a specified voltage between the supply voltages. The middle sub-range extends from the first specified voltage to a second specified voltage between the first specified voltage and the second supply voltage, and the second end sub-range extends from the second specified voltage to the second supply voltage.
To operate over the different ranges, Huijsing requires a switching of the operation currents depending on the differential portion used as the common mode voltage, VCM, enters the end range, where the input transistors in that differential portion are non-conductive. Further, the switching of the operating currents is purportedly done with one or more steering transistors differentially configured with respect to one or both pairs of input transistors. The supply lines for the other differential portion are then purportedly provided with current that is derived from current diverted away from the supply lines for one of the differential portions.
The switching involved in Huijsing, however, is undesirable in that it typically damages the common-mode CMRR. This occurs because the NMOS and PMOS transistors of the input stages have different offset voltages which require the switching point amplifier to rapidly change the offset voltage. The consequence of the rapid switching by the amplifier results in a low CMRR.
Accordingly, a need exist to provide a differential amplifier with a rail-to-rail input stage which provides a wide common-mode range, does not involve added complexities or require excessive switching, and additionally operates within very low voltage (e.g., 0.9-3.6V). supply range without damaging the gain of the differential amplifier input stage.
The method and circuit according to various aspects described herein addresses many of the shortcomings of the prior art. As discussed above, previous approaches for extending the operational range of the differential amplifiers with rail-to-rail input stage over the common-mode input voltages involve, inter alia, complex manufacturing techniques and/or have difficulty in maintaining a stable CMRR and input stage gain due to, complex switching techniques. However, in accordance with various aspects of the present invention, an operational amplifier is configured using a rail-to-rail input stage with single input pair, which avoids the problems found in the prior art.
The input stage transistors may be biased in accordance with the value of the common mode voltage. For example, where the common mode voltage VCM nears the low voltage rail, the bodies of the input transistors are biased by the voltage drop across a diode structure and the transistor base to source voltage, Vbs. Contrarily, where the common mode voltage VCM is near the high voltage rail, the base to source voltage Vbs value is at a value near the low voltage rail which provides sufficient headroom for the operation of the attached load or next amplifier stage.
In accordance with one aspect of the present invention, an operational amplifier with a differential input stage is provided which operates on a voltage supply of from approximately 0.9 Volts with a high common-mode rejection rate.
In accordance with one exemplary embodiment of the present invention, an amplifier with rail-to-rail differential input stage uses a single input NMOS differential pair. The common mode voltage value is provided to the body nodes of the NMOS differential pair which uses input common-mode voltage feedback to control the input differential pair body voltage. In this way, the relationship of the body-bias of the differential NMOS pair may be exploited to reduce the effective headroom of the differential pair and increase the common mode input voltage range.
In according with another aspect of the present invention, a body-bias voltage is used to control the threshold of the MOS transistors, where the body-bias voltage is dependent on the common-mode voltage of the transistor.